Near-eye displays, particularly those that also provide see-through views of the ambient environment, incorporate powered optics for forming virtual images without unduly obstructing views of the ambient environment. Some such displays fold light paths to the powered optics out of the line of sight to the ambient environment and in others, the powered optics are at least partially transparent to light from the ambient environment.
The powered optics folded out of the line of sight generally add to another dimension of the displays, particularly to the thickness of the displays. Such additional thickness is often undesirable. Near-eye displays are generally formed as thin as possible to more closely replicate the styles of other eyewear.
The powered optics that are located along the line of sight are at least partially transmissive to ambient light but are generally oriented off axis to improve overall light efficiency and to reduce thickness requirements of other display optics for directing image-bearing light to the powered optics. The focusing power of the powered optics is expressed under reflection. To preserve a natural view of the ambient environment, either an additional optic is required to undo the focusing power under transmission or the powered optic is formed as a holographic optical element that transmits ambient light largely undisturbed. The off-axis orientation of the in-line powered optics often requires corrections for both image distortion and chromatic aberration.